vscode-web.md: Add sentence about Firefox

[mb21]

[ntrogh]

@joaomoreno Can you review this docs PR for vscode.dev?

[joaomoreno]

@mb21 Can you show me exactly what happens if you don't exempt vscode.dev? I just tried it on my machine: I set the Protection Settings to Strict, and vscode.dev still works just fine.

[mb21]

@joaomoreno I believe it only fails if your GitHub account isn't linked to vscode.dev yet. So to test it you'd need to remove the 'app' from your GitHub profile. Then the first time it gives the error:

unable to resolve workspace folder (ENOPRO: No file system provider found for resource 'vscode:vfs://....')

See e.g. https://www.reddit.com/r/Floorp/comments/1gbymgr/comment/m3rexzg/?context=3

[mb21]

@joaomoreno or anybody wants to merge this?